by
Hall, Prentice

Answer

$r^{4} - 9s^{2}$

Work Step by Step

We use the formula for the difference of squares to apply to this question.
The difference of squares formula is:
$(a-b) (a+b) = a^{2} - b^{2}$
In the given formula let $r^{2}$ represents a and 3s represents b.
$(r^{2}-3s) (r^{2}+3s) = (r^{2})^{2} - (3s)^{2}$
= $r^{4} - 9s^{2}$